Dc-dc converter with reduced input current ripples

ABSTRACT

An improved DC-DC converter with reduced input current ripples. The converter includes a transformer, a switch, and a capacitor. The switch turns on and off alternately to store the energy in the transformer. The capacitor is connected in series with a rectifier and a secondary winding of the transformer across an input DC voltage so as to be charged by the energy released from the secondary winding through the DC power source. Accordingly, the circuit sees an input current which continues flowing through the DC power source even the switching element is turned off.

TECHNICAL FIELD

[0001] The present invention relates to a DC-DC converter, and moreparticularly to the DC-DC converter capable of reducing input currentripples for improving circuit efficiency.

BACKGROUND ART

[0002] U.S. Pat. No. 5,910,712 discloses a DC-DC converter of the typeknown as a fly-back converter which includes a transformer with aprimary winding connected in series with a switching element across aninput DC power source and a secondary winding connected across asmoothing output capacitor that is responsible for supplying an outputDC voltage to a load. In operation, the switching element is controlledto turn on and off for repetitively interrupting the input DC voltagesupplied to the primary winding so as to accumulate the energy in theprimary winding when the switching element is on and release thecorresponding energy from the secondary winding to charge the smoothingoutput capacitor when the switching element is off, thereby providing asmoothed DC output voltage to the load. Thus, it is possible to set theoutput DC voltage at a desired level even lower than the input DCvoltage by selecting a duty cycle of the switching element.

[0003] This circuit, however, permits no input current being suppliedfrom the input DC power source while the switching element is off,thereby suffering from increased input current ripples. The increasedripples results in lowering the circuit efficiency as well ascorresponding increased input current peak which necessitates a largecapacity for the transformer with attendant increase in the bulk of thetransformer. Also, since the transformer in this circuit is aloneresponsible for conveying the energy from the input DC power source tothe load, the transformer has to include a relatively large core inorder to prevent magnetic flux saturation and is therefore made into alarge bulk. Thus, it is difficult to use the transformer of compactdesign and to assemble the whole circuit into a compact sufficient to beinstalled within a limited space.

DISCLOSURE OF THE INVENTION

[0004] In view of the above insufficiency, the present invention hasbeen accomplished to provide an improved DC-DC converter with reducedinput current ripples which is capable of utilizing a compacttransformer for reducing the physical dimensions of the converter, aswell as to provide a ballast for a discharge lamp making the use of thecompact DC-DC converter. The DC-DC converter in accordance with thepresent invention has a converter input which is adapted to receive aninput DC voltage, and a converter output which is adapted to beconnected to a load for proving an output DC voltage to the load. Theconverter includes a transformer having a primary winding and asecondary winding. The primary winding is connected in series with aswitching element across the converter input. The switching element isdriven to turn on and off in order to repetitively interrupt the DCinput voltage and induce an energy at the secondary winding in responseto the switching element being turned off. A capacitor is connected incircuit to be charged by the energy released from the secondary windingso as to accumulate the output DC voltage, and is connected across theconverter output to provide the resulting output DC voltage to the load.

[0005] The characterizing feature of the present invention resides inthat the capacitor is connected in series with a rectifier and thesecondary winding so as to be charged by the energy released from thesecondary winding through the converter input. With this arrangement,the circuit sees an input current which continues flowing through theconverter input even while the switching element is turned off. Thus, nointerruption in the input DC current is assured to thereby reduce theinput current ripples and therefore the input current peak which enablesthe use of small-sized transformer for overall compact arrangement ofthe converter, yet improving the circuit efficiency.

[0006] A controller is included in the converter to determine aswitching frequency of the switching element that is sufficiently higherthan a resonance frequency given to a resonant system given by thecapacitor and the secondary winding in order to restrain undesirableresonance for reliable converter operation.

[0007] In one embodiment of the present invention, the capacitor isconnected in series with the primary winding across the DC power sourceso as to form a closed loop of the capacitor, the secondary winding, therectifier, the converter input and the primary winding for flowing theinput current therethrough while the switching element is off. For thispurpose, the secondary winding has a polarity chosen in relation to thewinding sense of the primary winding such that the input DC voltage issuperimposed in phase upon the voltages induced at the first and secondwindings, respectively.

[0008] In another embodiment of the present invention, the capacitor isconnected in series with the secondary winding and the rectifier acrossthe DC power source in parallel with a series combination of the primarywinding and the switching element. Thus, there is established a closedloop of the capacitor, the secondary winding, the rectifier and theconverter input for flowing the input current therethrough while theswitching element is off. To this end, the secondary winding has apolarity chosen in relation to the winding sense of the primary windingsuch that the input DC voltage is superimposed in phase upon the voltageinduced at the secondary winding. Alternatively, the secondary windingmay have a polarity chosen in relation to the winding sense of theprimary winding such that the input DC voltage is superimposed inreverse phase upon the voltage induced at the secondary winding.

[0009] Preferably, a low-pass filter is connected across the converteroutput in order to remove output ripples.

[0010] The DC-DC converter can be best applied to a ballast for adischarge lamp in which an inverter is connected to convert the outputDC voltage from the converter into an AC voltage for operating thedischarge lamp.

BRIEF DESCRIPTION OF THE DRAWINGS

[0011] A more complete appreciation of the invention and many of theattendant advantages thereof will become readily apparent with referenceto the following detailed description, particularly when considered inconjunction with the accompanying drawings, in which

[0012]FIG. 1 is a circuit diagram of a DC-DC converter in accordancewith a first embodiment of the present invention;

[0013]FIGS. 2A and 2B are diagram illustrating the operation of theconverter;

[0014]FIG. 3 is a waveform chart explaining the operation of theconverter;

[0015]FIG. 4 is a circuit diagram of a ballast for a discharge lampincorporating the above converter and an inverter;

[0016]FIG. 5 is a circuit diagram of another ballast for a dischargelamp incorporating a modified converter and the like inverter;

[0017]FIG. 6 is a circuit diagram of a DC-DC converter in accordancewith a second embodiment of the present invention;

[0018]FIGS. 7A and 7B are diagram illustrating the operation of theconverter;

[0019]FIG. 8 is a waveform chart explaining the operation of theconverter;

[0020]FIG. 9 is a circuit diagram of a DC-DC converter in accordancewith a second embodiment of the present invention;

[0021]FIGS. 10A and 10B are diagram illustrating the operation of theconverter; and

[0022]FIG. 11 is a waveform chart explaining the operation of theconverter.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0023] The preferred embodiments will now be described with reference tothe accompanying drawings, wherein like reference numerals designatecorresponding or identical elements throughout the various drawings.

[0024] Referring now to FIG. 1, there is shown a DC-DC converter inaccordance with a first embodiment of the present invention. A converter20 is adapted to be connected to a DC power source 10 to be suppliedwith an input DC voltage therefrom and provides a regulated output DCvoltage to energize a load 60. Although the illustrated embodiment showsthe DC power source 10 which includes a battery 12 and an input filtercomposed of an inductor 14 and a capacitor 16, the converter can beapplied to various DC power sources of different configurations. Theconverter 20 includes a transformer 30 having a primary winding 31 whichis connected in series with a switching element 24 (e.g. transistor)across input terminals 21, and a secondary winding 32 which givesantipolarity to the primary winding and is connected in series with arectifier, i.e., diode 26 in a forward bias relation across outputterminals 27. A smoothing output capacitor 34 is connected in serieswith the primary winding 31, the secondary winding 32, and the diode 26across the input terminals 21 with the capacitor 34 connected betweenthe windings 31 and 32. Connected between the output terminals 27 of theconverter 20 and the load 60 is a low-pass filter 40 composed of aninductor 41 and a capacitor 42. The switching element 24 is driven by acontroller 50 to turn on and off at a high frequency of, for example, 20kHz or more, and at a variable duty cycle based upon a detected voltageacross the load 60 and a detected load current I_(out) in order toprovide a constant output DC voltage to the load 60.

[0025] In operation, when the switching element 24 is on, an inputcurrent I_(in1) is drawn from the DC power source 10 to flow through theprimary winding 31 of the transformer 30, as indicated by an arrowedsolid line in FIG. 2A, so as to store the energy thereat. Uponsubsequent turn off of the switching element 24, the transformer 30releases its energy to flow an input current I_(in2) through a closedloop of the secondary winding 32, diode 26, DC power source 10, primarywinding 31, and capacitor 34, as indicated by an arrowed solid line inFIG. 2B, so as to charge the smoothing output capacitor 34. The currentI_(in2) includes a current drawn from the DC power source 10 so as togive no interruption in the input current from the DC power source whilethe switching element is off, thereby reducing the input current ripplesand therefore the current peak. When the switching element issubsequently on, the smoothing output capacitor 34 discharges to flow anoutput current I_(out1) through the switching element 24, as indicatedby an arrowed broken line in FIG. 2A, for energizing the load. In thissense, the smoothing output capacitor 34 can be defined as an outputcapacitor which provides the output DC voltage to the load when theswitching element 24 is on. When the switching element is off, the loadis continuously supplied with an output current I_(out2) which isreleased from the secondary winding 32 to flow through the diode 26, asindicated by an arrowed broken line in FIG. 2B. As indicated by dots inthe figures, the secondary winding 32 has a winding sense in relation tothe primary winding 31 such that the input DC voltage from the DCvoltage source is superimposed in phase upon the voltages of the primaryand secondary windings 31 and 32 when the switching element 24 is off.

[0026]FIG. 3 shows various currents flowing through the circuit of theconverter which demonstrates the above circuit operations. In thisfigure, an input current, which is a combination of currents I_(in1) andI_(in2), is expressed by a current I₃₁ flowing through the primarywinding 31, while the output current I_(out) is a combination of thecurrents I_(out1) and I_(out2) flowing through the inductor 41. CurrentI₃₂ denotes a current flowing through the secondary winding 32.

[0027] As explained in the above, the input current is continuously fedfrom the DC power source 10 irrespectively of the on/off condition ofthe switching element 24, the converter can successfully reduce theinput current ripples and therefore the input current peak. This reducesa power requirement to the transformer 30 and therefore makes itpossible to use the transformer of a compact size. In addition, sincethe primary winding 31 is connected in series with the secondary winding32 and is cooperative therewith to charge the smoothing output capacitor34 when the switching element is off, as shown in FIG. 2B, the number ofturns of the primary winding 31 is additive to that of the secondarywinding 32 in the function of charging the smoothing output capacitor34. This means that the secondary winding 32 can be made to have reducednumber of turns by the corresponding number of turns of the primarywinding. With this result, the transformer can be further made compactas compared to a case in which the secondary winding is alone forcharging the capacitor 34. In addition, the reduced current ripple makesit possible to use the capacitor 34 of less capacitance while retainingthe intended function, thereby assisting to make the whole assemblycompact.

[0028]FIG. 4 shows a ballast for a discharge lamp as one typicalapplication of the DC-DC converter with the load being configured toinclude an inverter 70 providing a low frequency AC voltage of 1 kHz orless for operating the lamp 100, and a starter 80 providing a highstarting voltage of 20 kV or more to the lamp. The inverter 70 has fourswitching transistors 71 to 74 arranged in the form of a full-bridgeconnection. The transistors are driven by a driver 76 such that adiagonally opposed pair of transistors 71 and 74 are simultaneouslyturned on and off in an alternate relation to the other pair oftransistors 72 and 73, thus converting the output DC voltage from theconverter into the AC voltage being applied to the lamp 100. The driver76 is connected to receive a low frequency control signal of 1 kHz orless from the controller 50′ to make the low frequency inverter output.The starter 80 includes a transformer with a primary winding 81 and asecondary winding 82 which is connected in series with the lamp 100 in apath of feeding the inverter output. Connected across the primarywinding 81 is a series combination of a capacitor 84 and a switch 85which is responsible for discharging the capacitor 84 so to induce thehigh starting voltage at the secondary winding 82 for applying it tostart the lamp.

[0029] The capacitor 84 is charged by a booster 90 which makes the useof a voltage appearing in the secondary winding 32 to provide a boostedDC voltage sufficient for rapidly charging the capacitor 84. The booster90 is configured as a Cockcroft rectifier composed of diodes 91 to 94,capacitors 95 to 98, and a resistor 99. The booster 90 has its inputconnected across the diode 26 of the converter 20 and generates theboosted DC voltage from the voltage across the diode 26.

[0030]FIG. 5 shows another ballast which is identical to that of FIG. 4except that the converter 20 is somewhat modified such that the booster90 derives a voltage appearing across a series combination of thecapacitor 24 and the diode 26. In this connection, the diode 26 isconnected in series between the secondary winding 32 and the smoothingoutput capacitor 34, while the converter 20 retains the same operationsas discussed in the above. Like parts are designated by like numeralsfor an easy reference purpose.

[0031]FIG. 6 shows a DC-DC converter 20A in accordance with a secondembodiment of the present invention which is similar to the firstembodiment except that the smoothing output capacitor 34A is connectedin series with the secondary winding 32A and the diode 26A across the DCpower source 10 in parallel relation to a series combination of theprimary winding 31A and the switching element 24A. Like parts aredesignated by like reference numerals with a suffix letter of “A”. Aswill be explained below, the secondary winding 32A has the same polarityas the primary winding 31A such that the input DC voltage issuperimposed in phase upon the voltage of the secondary windings 32Awhen the switching element 24A is off.

[0032] Operation of the converter is explained with reference to FIGS.7A and 7B. When the switching element 24A is on, DC power source 10supplies an input current I_(in1) flowing through the primary winding31A, as indicated by an arrowed solid line in FIG. 7A, to store theenergy at the transformer 30A. Upon subsequent turn oft of the switchingelement 24A, the secondary winding 32A releases its energy to flow anoutput current I_(out2) through the diode 26A to the load, as indicatedby an arrowed broken line in FIG. 7B, while allowing an input currentI_(in2) to continue flow from the DC power source 10 through thesmoothing output capacitor 34A, the secondary winding 32A and the diode26A, as indicated by an arrowed solid line in the same figure, therebycharging the smoothing output capacitor 34A. Upon subsequent turn on ofthe switching element 24A, the smoothing output capacitor 34A thuscharged is made responsible for flowing an output current I_(out1)through the primary winding 31A and the switching element 24A to theload as indicated by a broken line in FIG. 7A. Also, the smoothingoutput capacitor 34A is responsible for flowing an output currentI_(out3) through the DC power source 10 to the load, as indicatedanother broken line, while the switching element 24A is off.

[0033]FIG. 8 shows various currents flowing in the circuit of theconverter for demonstrating the above circuit operations. In thisfigure, an input current I_(in) is a combination of currents I_(in1),I_(in2), and I_(out3), while the output current I_(out) is a combinationof the currents I_(out1), I_(out2), and I_(out3) supplied to the load.Currents I₃₁ and I₃₂ denote those flowing respectively through theprimary and secondary windings 31A and 32A. As confirmed by the waveformof FIG. 8, the converter of this embodiment also assures no interruptionin the input current for reducing the input current ripples as well asreducing the input current peak.

[0034]FIG. 9 shows a DC-DC converter 20B in accordance with a thirdembodiment of the present invention which is identical to the secondembodiment except that the secondary winding 32B has its winding sensechosen oppositely to that of the second embodiment for flowing input andoutput currents through the secondary winding 32B and the diode 26B in areverse direction. Like parts are designated by like reference numeralswith a suffix letter of “B”.

[0035] Operation of the converter is explained with reference to FIGS.10A and 10B. When the switching element 24B is on, DC power source 10supplies an input current I_(in1) flowing through the primary winding31B, as indicated by an arrowed solid line in FIG. 10A, to store theenergy at the transformer 30B. Upon subsequent turn off of the switchingelement 24B, the secondary winding 32B releases its energy to flow acurrent I_(out1) through the diode 26B to the load, as indicated by anarrowed broken line in FIG. 10B. At the same time, the smoothing outputcapacitor 34B is cooperative with the DC power source 10 to flow anadditional current I_(out2) to the load. Since the current I_(out2)flows through the DC power source 10, it can be regarded as an inputcurrent which continues to flow, even in the off-period of the switchingelement 24B, from the DC power source 10 to the converter, therebyreducing the input current ripples and the input current peak as is madein the previous embodiment. It is noted that during the on-period of theswitching element 24B, the DC power source 10 is also responsible forflowing a like current I_(out3) through the smoothing output capacitor34B to the load.

[0036]FIG. 11 shows various currents flowing in the circuit of the aboveconverter 20B for demonstrating the above circuit operations. In thisfigure, an input current I_(in) is a combination of currents I_(in1),I_(in2), I_(out2), and I_(out3), while the output current I_(out) is acombination of the currents I_(out1), I_(out2), and I_(out3) supplied tothe load. Currents I₃₁ and I₃₂ denote those flowing respectively throughthe primary and secondary windings 31B and 32B. Again as confirmed bythe waveform of FIG. 11, the converter of this embodiment also gives nointerruption in the input current for reducing the input current ripplesas well as reducing the input current peak.

[0037] This application is based upon and claims the priority ofJapanese Patent Application No. 2000-348758, filed in Japan on Nov. 15,2000, the entire contents of which are expressly incorporated byreference herein.

1. A DC-DC converter comprising: a converter input which is adapted toreceive an input DC voltage; a converter output which is adapted to beconnected to a load for providing an output DC voltage to said load; aswitching element which is connected across said converter input and isdriven to turn on and off; a transformer having a primary winding and asecondary winding, said primary winding being connected in series withsaid switching element across said converter input to induce an energyat said secondary winding in response to said switching element beingturned off, a capacitor which is connected in circuit to be charged bysaid energy released from said secondary winding so as to accumulatesaid output DC voltage, and which is connected across said converteroutput to provide said output DC voltage to said load, wherein saidcapacitor is connected in series with a rectifier and said secondarywinding across said converter input so as to be charged by the energyreleased from said secondary winding through said converter input andsaid rectifier.
 2. The DC-DC converter as set forth in claim 1, furtherincluding: a controller which determines a switching frequency of saidswitching element that is sufficiently higher than a resonance frequencygiven to a resonant system given by said capacitor and said secondarywinding so as to restrain the resonance.
 3. The DC-DC converter as setforth in claim 1, wherein said capacitor is connected in series withsaid primary winding across said converter input.
 4. The DC-DC converteras set forth in claim 3, wherein said secondary winding has a polaritychosen in relation to the winding sense of the primary winding such thatthe input DC voltage is superimposed in phase on the voltage induced atsaid primary and secondary windings.
 5. The DC-DC converter as set forthin claim 1, wherein said capacitor is connected in series with saidsecondary winding and said rectifier across said converter input inparallel with a series combination of said primary winding and saidswitching element.
 6. The DC-DC converter as set forth in claim 5,wherein said secondary winding has a polarity chosen in relation to thewinding sense of the primary winding such that the input DC voltage issuperimposed in phase upon the voltage induced at the secondary winding.7. The DC-DC converter as set forth in claim 5, wherein said secondarywinding has a polarity chosen in relation to the winding sense of theprimary winding such that the input DC voltage is superimposed inreverse phase upon the voltage induced at the secondary winding.
 8. TheDC-DC converter as set forth in claim 1, wherein a filter is connectedacross said converter output so as to remove output ripples.
 9. TheDC-DC converter as set forth in claim 8, wherein said filter is alow-pass filter.
 10. A ballast for a discharge lamp, said ballast beinga combination of a DC-DC converter and an inverter, said DC-DC convertercomprising a converter input which is adapted to receive an input DCvoltage; a converter output which is adapted to be connected to a loadfor providing an output DC voltage to said load; a switching elementwhich is connected across said converter input and is driven to turn onand off; a transformer having a primary winding and a secondary winding,said primary winding being connected in series with said switchingelement across said converter input to induce an energy at saidsecondary winding in response to said switching element being turnedoff, a capacitor which is connected in circuit to be charged by saidenergy released from said secondary winding so as to accumulate saidoutput DC voltage, and which is connected across said converter outputto provide said output DC voltage to said load, wherein said capacitoris connected in series with a rectifier and said secondary windingacross said converter input so as to be charged by the energy releasedfrom said secondary winding through said converter input and saidrectifier, said inverter being connected to said converter output andconverting said output DC voltage into an AC voltage for operating thedischarge lamp.